Plasmon is a vibrational quantum of free electrons in materials such as metal. For example, when nano-scale fine metal (metal nanostructure) is irradiated with light, collective oscillation of the free electrons happens to the metal due to the light. Consequently, the plasmon is excited.
As a result of the excited plasmon, an electromagnetic field with locally high intensity is generated. Energy of the resultant electromagnetic field from the excitation of plasmon may be potentially used for various purposes.
Technologies to optically record information at very high density are known as the potential usage (c.f. Patent Document 1). As described above, light entering a scattering substance excites the localized plasmon. The excitation of the localized plasmon locally intensifies an optical electric field near the scattering substance. Light near the scattering substance is used to record information in a minute area in the order of nanometers outside the diffraction limit.
FIG. 26 is a schematic view of a conventional optical pickup device 900 disclosed in Patent Document 1. The conventional optical pickup device 900 is described with reference to FIG. 26.
The optical pickup device 900 uses plasmon to record or reproduce information on or from a recording medium 910. The optical pickup device 900 includes a laser beam source 920, a mirror 930, a lens 940, a substrate 950 and a scattering substance 960. The laser beam source 920 emits a laser beam LB toward the mirror 930. The mirror 930 reflects the laser beam LB toward the lens 940. The lens 940 focuses the laser beam LB toward the substrate 950.
The substrate 950 is formed from a transparent material to the laser beam LB. The substrate 950 includes a first facing surface 951, which faces the lens 940, and a second facing surface 952, which faces the recording medium 910. The scattering substance 960 is formed on the second facing surface 952. The laser beam LB passing through the lens 940 goes through the first facing surface 951, and then is focused on the scattering substance 960.
The substrate 950 moves above the recording medium 910. The scattering substance 960 is embedded in the second facing surface 952. Consequently, the scattering substance 960 does not interfere with the movement of the substrate 950 above the recording medium 910. According to Patent Document 1, the second facing surface 952 of the substrate 950 is provided with a cylindrical hole, which is approximately 50 nm in diameter and approximately 100 nm in depth. The scattering substance 960 is gold embedded in the cylindrical hole.
The optical pickup device 900 includes a position adjustment mechanism (not shown) to get the lens 940 closer to and away from the substrate 950. The position adjustment mechanism slightly moves the lens 940 vertically. The position adjustment mechanism adjusts and fixes a position of the lens 940 so that the focal point of the laser beam LB coincides with the center of the scattering substance 960.
Localized plasmon is excited around the scattering substance 960 under appropriate positional adjustment to the lens 940 if the laser beam LB enters the scattering substance 960. Consequently, the optical electric field is intensified near the scattering substance 960.
According to Patent Document 1, the scattering substance 960 is a cylindrical minute metal. The bottom surface of the scattering substance 960 is substantially flush with the second facing surface 952, which faces the recording medium 910. Accordingly, the intensified optical electric field covers in an area, which substantially corresponds to the bottom surface of the scattering substance 960 (i.e. circular area which is approximately 50 nm in diameter).
The optical pickup device 900 may be mounted on a recording device or a reproduction device. The recording or reproduction device may have functions for controlling a distance between the scattering substance 960 and the recording medium 910. When the scattering substance 960 gets closer to be a predetermined distance from the recording medium 910, information (hereinafter referred to as “spot information”) is recorded in an area corresponding to the spot diameter of the area of the intensified optical electric field.
Less intensified light than the light used for the recording operation is utilized in order to reproduce the spot information recorded on the recording medium 910. The optical pickup device 900 detects the reflected light, which is reflected from the scattering substance 960, or the transmitted light, which is transmitted through the scattering substance 960. Consequently, information written in a minute area below the diffraction limit is reproduced.
The scattering substance 960, which is used in the conventional optical pickup device 900 shown in FIG. 26, is sized to be not greater than a wavelength of the laser beam LB, in order to satisfy conditions for generating localized plasmon resonance Since the size of the scattering substance 960 is several hundred nm or less, the reproduction light, which is used for reproducing the recorded information (reflected light which is reflected from the scattering substance 960 or transmitted light which is transmitted through the scattering substance 960), is very weak. Accordingly, the conventional optical pickup device 900 needs a very sensitive photoelectric detector in order to detect the weak reproduction light.
Optical devices such as photo-multipliers and avalanche photodiodes (APD) are known as highly sensitive photoelectric detectors. With regard to photo-multipliers, an oversized element may be a problem. Accordingly, it is physically difficult to manufacture a practical optical pickup device by using a photo-multiplier. In addition, since elements of a photo-multiplier are expensive, it is also difficult to manufacture a practical optical pickup device in terms of manufacturing costs. If an avalanche photodiode is used, a temperature control system to reduce temperature drift of elements and a control circuit to handle high voltages are required. Consequently, the use of an avalanche photodiode complicates a reproduction light detection system.
As described above, the lens 940 focuses light toward the scattering substance 960 to cause a light focusing spot. Due to the diffraction limit of light, the lens 940 may not form a light focusing spot smaller than the wavelength of the light in principle. Accordingly, the light focusing spot may be larger than the scattering substance 960.
As a result of the larger light focusing spot than the scattering substance 960, even when the lens 940 focuses light on the scattering substance 960, there may be light components beyond the scattering substance 960. The light components beyond the scattering substance 960 are scattered or reflected around the scattering substance. Accordingly, the light components beyond the scattering substance 960 may be superimposed as noise light on the reflected light from the scattering substance 960, which is used for signal reproduction. As described above, since noise light is superimposed on the very weak reproduction light, it is very difficult to obtain reproduction signals excellent in a signal/noise ratio (SN ratio), by means of the conventional optical pickup device 900 which utilizes plasmon.
The aforementioned problems are common among various devices which utilize plasmon resonance.
Patent Document 1: JP 2002-8235 A